The Yul optimizer causes mstore results to be inconsistent.

[Subway2023]

Environment

• Compiler version: 0.8.26
• Target EVM version (as per compiler settings): None
• Framework/IDE (e.g. Truffle or Remix): None
• EVM execution environment / backend / blockchain client: None
• Operating system: Linux

Steps to Reproduce

{
    function f(a, b) -> x { 
        x := add(b, a) 
    }
    function g() -> y { 
        y := mload(0) 
        mstore(0, 4) 
    }
    let i := 0
    for {} lt(i, 2) { i := add(i, 1) } 
    {
        let x := 1337
        if lt(i, 1) {
            x := 42
            continue
        }
        mstore(0, x)
    }
    let r := f(g(), mload(0))
    sstore(0, r)
}

Get Bin

normal

solc-0826  --strict-assembly --bin test.yul
5f5b60028110601a5760165f5160126037565b6033565b5f55005b600190610539828210602e575f525b016001565b506029565b0190565b5f519060045f5256

optimize

solc-0826  --strict-assembly --optimize --bin test.yul
5f5b600281106010575f5180015f55005b6001906105398282106024575f525b016001565b50601f56

Run in EVM

normal

go-ethereum/build/bin/evm --debug --json --code 5f5b60028110601a5760165f5160126037565b6033565b5f55005b600190610539828210602e575f525b016001565b506029565b0190565b5f519060045f5256 run

optimize

go-ethereum/build/bin/evm --debug --json --code 5f5b600281106010575f5180015f55005b6001906105398282106024575f525b016001565b50601f56 run

Execution result analysis

normal

output,storage,memory
{'output': '', 'gasUsed': '0x575c'},{'0': '2674'},{'0': '4'}

optimize

output,storage,memory
{'output': '', 'gasUsed': '0x571c'},{'0': '2674'},{'0': '1337'}

The data at position 0 is different. The suspected cause is that mstore(0, 4) was incorrectly optimized away.